1. Field of the Invention
The present invention relates to a bump electrode and a method for fabricating the same, and more particularly, to a bump electrode for bonding the substrate to external terminals and a method for fabricating the same.
2. Discussion of the Related Art
A bonding method using a bump has been widely used in packaging of a semiconductor device or the driving device of a liquid crystal display (LCD). The driving device of a LCD is connected to a thin film transistor (TFT) array substrate. A technique of packaging the driving device, in general, has the following variations: (1) the driving device is mounted on a printed circuit board (PCB), and then the PCB and the TFT array substrate are connected to each other, (2) the driving device is mounted on a flexible tape, and the flexible tape and the TFT array substrate are connected to each other, (3) the driving device is directly mounted on the substrate (Chip on Glass; COG).
The bonding method using a bump is mainly used in a direct package technique. In this method, an anisotropic conductive film (ACF) or anisotropic conductive adhesive (ACA) is used to bond a device having the bump to the terminal of an external substrate. These ACF and ACA contain conductive balls which are 5 to 7 .mu.m large in diameter. When ACF or ACA is coated on a mounting portion of an external substrate and the device having the bump is pressed on the external substrate, the device and substrate are electrically connected to each other through the conductive balls contained in ACF or ACA. However, as the size of semiconductor device becomes smaller, the distance between the bumps formed on the device becomes shorter. Thus, the adjacent bumps can be electrically connected to each other through the conductive balls contained in ACF or ACA. This causes an electrical short in the device.
FIG. 1 is a cross-sectional view of a conventional substrate bump. The conventional bump electrode includes a pad 2 on a semiconductor substrate 1, on which a driving device is formed. A passivation layer 3 is formed on a portion of the pad 2 and an exposed portion of the substrate 1. A barrier metal 4 (used for diffusion stop, adhesive and oxidation resistance layer) is formed on an exposed portion of pad 2 and a portion of the passivation layer 3 over the pad 2. A bump 5 is formed on the barrier metal 4.
FIG. 2 shows a state in which adjacent bumps 5 are electrically connected to each other through conductive balls 6 contained in an adhesive material 7 when the semiconductor substrate 1 is bonded to a substrate 8. In the bonding process, an adhesive material 7, such as ACF or ACA, is coated on the substrate 8, and then heat and pressure are applied to the semiconductor substrate 1 to thereby bond the substrate 1 to the substrate 8. Here, according to heat or pressure applied to the bump 5, the adhesive material 7 flows between the bumps, and gathers the conductive balls in the space between the bumps. In this way, the adjacent bumps can be electrically connected to each other.
The top surface of the bump 5 actually comes into contact with the terminal of the external substrate 9. However, since the side of the bump 5 is exposed, if the bumps are close enough to each other, the conductive balls contained in the adhesive material may gather between the bumps and electrically connect the adjacent bumps. This causes an undesirable electrical short between the adjacent bumps.